This invention relates to a brake assembly particularly for a retracting sootblower for boiler cleaning having an indexing lance tube drive system.
Sootblowers are used to project a stream of blowing medium such as water, air or steam against heat transfer surfaces within large scale boilers to cause slag and ash encrustations to be removed. The blowing medium impact produces mechanical and thermal shock which causes these adhering layers to be removed. One general category of sootblowers is known as the long retracting type. These devices have a retractable lance tube which is periodically advanced into and withdrawn from the boiler and is simultaneously rotated such that one or more nozzles at the end of the lance tube project jets of blowing medium tracing helical paths.
Many conventional sootblowers, such as the so-called "IK" sootblower manufactured by Applicant, include a lost motion device which causes the nozzles to return along a helical path that bisects the helical path traced during forward travel. This indexing enables surfaces that were not cleaned during extension to be subjected to blowing medium upon retraction. Although the lance tube nozzles trace different helical paths upon extension and retraction, the positions of these helical paths are nonetheless fixed. Heat transfer surfaces continually subjected to impact by blowing medium suffer from erosion and wear. Furthermore, areas lying between the helical paths of the nozzle jets can sometimes escape adequate cleaning. In view of the foregoing, there is a need to provide a long retracting sootblower device having an indexing mechanism which provides a large number of different yet predictable helical paths traced by the lance tube nozzles.
U.S. Pat. No. 4,803,959, commonly assigned to the assignee of the present application, discloses a sootblower having an indexing drive system which slightly displaces the helical paths traced by the nozzles upon each sootblower actuation cycle, and thus overcomes the above mentioned disadvantages of prior art designs. The system according to the patent is particularly adapted for sootblowers having a lance tube driving carriage which is advanced and retracted while simultaneously rotating the lance tube. For many long retracting sootblower designs, the carriage has an electric drive motor which is coupled to a drive train having one or more pinion gears which mesh with elongated gear racks to cause the carriage and the lance tube to be retracted and advanced. The carriage also drives the lance tube for rotation through bevel gears.
The indexing system of the above patent provides for the displacement of the driving pinion gear along the longitudinal drive racks. The indexing system causes the pinion gear meshing with the rack to advance a predetermined number of gear teeth, with respect to the rack, upon each actuation cycle. Due to the geared connection between the lance tube's longitudinal and rotational drive systems, such drive pinion advancement causes a slight change in the position of the helical paths traced by the lance nozzles upon each actuation cycle. In other words, the position of the helical paths traced by the lance nozzles will be indexed a position or distance corresponding to the advancement of pinion. The indexing mechanism of the prior patent employs a movable rack segment which meshes with the main rack element and is caused to index by the tractive effort exerted by the pinion drive gear as the lance tube begins its cleaning cycle. In this manner, the indexing mechanism operates automatically and has been incorporated into new sootblowers as well as many existing sootblowers as a retrofit application.
Although the indexing mechanism of the above patent has been shown to operate well for numerous applications, it has some limitations when an attempt is made to apply it to certain new and retrofitting applications. As more fully described below, the application limitation results in cases where the carriage begins its initial forward movement without an adequate load being provided on the driving pinion shaft. Without the initial tractive effort, the pinion fails to index the movable rack segment and fails to be advanced relative to rack. Thus, the patent's indexing mechanism fails to be activated. In the present invention, a spring-loaded brake or friction key assembly is provided which will impede forward movement of the carriage until the load required to advance the pinion relative to the rack has been produced and the pinion has been advanced.
While the brake assembly was designed for incorporation with the indexing mechanisms of sootblowers, the invention of this application will also find utility wherever forward movement needs to be delayed until the development of a specific driving force.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.